Within various animal species retroviruses present in the form of infectious agents are responsible for tumor development. These retroviruses can also be found in the form of endogenous proviral genomes present in the germline of all vertebrates, including man. The C3Hf inbred mouse has provided an animal model for the study of the involvement of endogenous retroviral genes of the Mouse Mammary Tumor Virus (MMTV) in the development of breast cancer. Within the human genome DNA sequences have been detected under low stringency hybridization conditions which are homologous to MMTV. The aims of our proposal are to investigate the structure, function, and casual relationship of these human MMTV-like proviral genomes to human breast cancer using molecular probes which are in some cases unique to our laboratory. Human MMTV-like sequences will be isolated from a placental library and characterized in detail by restriction digestion and hybridization to probes of different regions of the MMTV genome. Cloned human sequences homologous to MMTV will be used as molecular probes to determine the pattern of human MMTV-like sequences in normal and malignant breast tissue. Our proposed study is to first of its type in which human probes containing MMTV-like sequences will be used to analyze a statistical number of breast cancers. DNA will be isolated from fresh breast tissue as well as from paraffin-embedded tissue, digested with restriction enzymes, electrophoresed through agarose gels, transferred to nitrocellulose paper, and hybridized to cloned human probes of MMTV-like sequences. We shall determine if MMTV-like sequences are amplified, rearranged, or deleted in human breast cancers. If additional MMTV-like sequences are present in malignant breast tumors we shall compare their site(s) of integration to a common proviral integration site which we have cloned from C3Hf tumors. To determine the transcriptional potential of human MMTV-like sequences we shall hybridize RNA isolated from normal and malignant tissue to probes of different regions of the MMTV genome and to probes of human MMTV-like sequences. In addition through constructs of human LTR sequences and expression vectors we shall determine if human MMTV-like LTRs can promote RNA transcription and if this transcription is affected by steroid hormones. In the C3Hf mouse only one out of 3 MMTV proviruses is transcribed. We shall compare the cellular sequences which flank the mouse and the human MMTV sequences to determine if similarities exist and if so if they might be correlated with the transcription potential of the human MMTV- like sequences.